Progress In Electromagnetics Research C, Vol. 79, 115–126, 2017 High Gain Slotted Waveguide Antenna Based on Beam Focusing Using Electrically Split Ring Resonator Metasurface Employing Negative Refractive Index Medium Adel A. A. Abdelrehim and Hooshang Ghafouri-Shiraz* Abstract—In this paper, a new high performance slotted waveguide antenna incorporated with negative refractive index metamaterial structure is proposed, designed and experimentally demonstrated. The metamaterial structure is constructed from a multilayer two-directional structure of electrically split ring resonator which exhibits negative refractive index in direction of the radiated wave propagation when it is placed in front of the slotted waveguide antenna. As a result, the radiation beams of the slotted waveguide antenna are focused in both E and H planes, and hence the directivity and the gain are improved, while the beam area is reduced. The proposed antenna and the metamaterial structure operating at 10 GHz are designed, optimized and numerically simulated by using CST software. The effective parameters of the eSRR structure are extracted by Nicolson Ross Weir (NRW) algorithm from the s-parameters. For experimental verification, a proposed antenna operating at 10 GHz is fabricated using both wet etching microwave integrated circuit technique (for the metamaterial structure) and milling technique (for the slotted waveguide antenna). The measurements are carried out in an anechoic chamber. The measured results show that the E plane gain of the proposed slotted waveguide antenna is improved from 6.5 dB to 11 dB as compared to the conventional slotted waveguide antenna. Also, the E plane beamwidth is reduced from 94.1 degrees to about 50 degrees. The antenna return loss and bandwidth are slightly changed. Furthermore, the proposed antenna offered easier fabrication processes with a high gain than the horn antenna, particularly if the proposed antenna is scaled down in dimensionality to work in the THz regime. 1. INTRODUCTION During the last two decades, metamaterials (MTMs) have received great attention due to their fascinating electromagnetic (EM) properties. MTMs are artificial atoms which exhibit exotic EM properties that cannot be achieved by the natural materials. MTMs can provide negative, zero or positive electric permittivity and magnetic permeability by inserting inclusion with specific geometrical shape and dimensionality in a host medium, and both of the inclusions and the host medium are constructed from metals and dielectrics [1]. There are two main categories for the MTMs which are resonance and non-resonance [2, 3], and each group has its own advantages and disadvantages and can be used to develop nowadays practical applications. Recently, MTMs with simultaneously negative permittivity, permeability and negative index of refraction are used to design perfect lenses [4, 5] for high resolution imaging system and to design many antennas [6, 7]. Also, MTMs are proposed to design gradient index of refraction lenses [8], absorbing, cloaking [9, 10], polarization transformer [11]. By using MTMs with negative or gradient index of refraction, compact size, high directive antennas Received 7 February 2017, Accepted 30 September 2017, Scheduled 5 November 2017 * Corresponding author: Hooshang Ghafouri-Shiraz ([email protected]). The authors are with the School of Electronic, Electrical and System Engineering, University of Birmingham, Birmingham, B15 2TT, United Kingdom. 116 Abdelrehim and Ghafouri-Shiraz can be designed [12, 13]. It is well know that high directive antennas play an important role in many practical applications. Many conventional efforts have been made to improve the antenna directivity such as parasitic patches, array of patch antennas, and parabolic reflectors [14–16]. Unfortunately, the aforementioned approaches suffer from large sizes, design methodology, complex feeding network and fabrication processes, particularly in the high frequencies. As Pendry investigated the first negative refractive index perfect lens (i.e., double negative MTMs with simultaneously negative permittivity, permeability and negative index of refraction) in 2000 [17], it is possible to design a compact size and high directive antennas [18, 19]. Furthermore, metamaterial structures composed of 3-D metal grid superstrates have been used to improve the directivity of the antennas [20, 21]. It should be noted that the aperture size of the metamaterial superstrate is much bigger than the size of the patch which makes the antenna very bulky. Also, a zero index medium metamaterial superstrate is used to develop a high gain and wideband antenna [22]. It is clear from the structure of the antenna that the metamaterial superstrate uses via to resemble shunt inductance, which makes it difficult in fabrication. The photonic crystal-based resonant antenna presented in [23] is a high directive antenna, but it is very bulky and has a narrow bandwidth. Engheta and his research team proved numerically and theoretically that a PEC screen with small hole covered by two subwavelength metamaterial structures can be used to improve the directivity of the antenna [24]. Planar metamaterial with NRI has been developed using split ring resonator (SRR) in [25]. This work focuses on two interesting contributions. Firstly, a 2D single sided metasurface structure based on an electrically split ring resonator (eSRR) [26] is designed by choosing proper dimensionality and orientation such that it exhibits NRI property in the direction of the wave propagation, and hence it can be used to improve the directivity of the EM emission. This 2D metasurface structure has the advantage of easier fabrication and assembly processes over the volumetric MTMs (mentioned in [20– 22]), so it can be scaled down to be fabricated for THz antennas for high resolution imaging and biomedical application (e.g., heart beat measurements). Secondly, a slotted waveguide antenna [27] is used as EM radiator instead of horn antenna or patch antenna because it is difficult to design a horn antenna with flared surfaces in the THz regime. Furthermore, there are no SMA connectors for the patch antenna operating in the THz regime. Fortunately, for waveguide, there are waveguide connectors up to 900 GHz [28, 29]. Thus the only solution to design a THz antenna is to use waveguide fed or photo conductive antennas. Here, we use the waveguide to feed the slotted antenna. Due to the low gain of the single element slotted antenna (the typical gain is 6.5 dB), NRI medium is used as a superstrate to improve the transverse electromagnetic (TEM) wave radiated from the slotted waveguide antenna, and hence the E and H plane gains and directivities are improved. For experimental verification, the slotted waveguide antenna incorporated with eSRR NRI medium operating at 10 GHz is designed, numerically optimized in CST software, fabricated and measured. This paper is organized as follows. Section 2 provides the design and simulation of a 10 GHz conventional slotted antenna. Section 3 discusses the design and effective parameter extraction of NRI medium based on eSRR. Section 4 presents the design and numerical simulation of the 10 GHz slotted waveguide antenna incorporated with NRI medium. In section 5, experimental verifications for the proposed antenna are carried out. Finally, the conclusion and future recommendations are given in section 6. 2. DESIGN AND SIMULATION OF 10 GHZ SLOTTED WAVEGUIDE ANTENNA According to Babinet’s principle, in order to form aperture radiator, a slot of length Ls and width Ws can be cut in the side wall, top wall or even in the back wall of a waveguide [30]. Here, a conventional 10 GHz slotted waveguide antenna is designed and simulated using Finite Element based Electromagnetic CST simulator. Fig. 1 illustrates the structure of the antenna; the antenna is constructed from WR-90 waveguide of standard dimensions a and b of 22.86 mm and 10.16 mm, respectively. The slot is cut in the back wall of the waveguide. The slot dimensions are designed properly such that the antenna operates at 10 GHz and has a good impedance matching with the free space; the slot length is about free space half wavelength. Three different slotted antennas of back wall slots of dimensions (Ls,Ws) of (14.5, 2), (14.7, 4), and (15.2, 6) are designed, and all dimensions are in mm. The waveguide has length Lwg of 30 mm which equals one free space wavelength at the operating frequency of 10 GHz and Progress In Electromagnetics Research C, Vol. 79, 2017 117 (a)(b) (c) Figure 1. Conventional waveguide slotted antenna, (a) 3-D view, (b) back view and (c) side view. (a) (b) Figure 2. Conventional 10 GHz slotted waveguide antenna performance at different slot dimensions of length Ls and width Ws, all dimensions are in mm, (a) return loss S11 in dB and (b) E-palne Gain in dB. thickness 3 mm to match the requirements of the fabrication, mechanical, and assembly processes. The antenna is simulated in CST software; the return loss S11 and gain are extracted as shown in Fig. 2. It is clear from Fig. 2 that when the slot width Ws increases, and the slot length Ls is changed slightly around the free space half wavelength of 15 mm at 10 GHz to keep the operating frequency at 10 GHz, the gain slightly decreases, while the bandwidth increases. Furthermore, the results illustrate that for slotted waveguide antennas of slot widths Ws of 2, 4, and 6 mm and slot lengths Ls of 14.5, 14.7 and 15.2 mm, the bandwidths are 0.72, 1.5 and 2.88 GHz, respectively, and the gain is about 6.5 dB mostly for all the three slot dimensions. 3. DESIGN AND SIMULATION OF 10 GHZ NEGATIVE REFRACTIVE INDEX LAYER BASED ON ESRR Here, a unit cell of an Electrically Split Ring Resonator (eSRR) metasurface structure is designed and simulated in CST. The eSRR unit cell is optimized and repeated in two dimensions which may provide electric and magnetic plasmonic resonances around 10 GHz and hence exhibits simultaneous negative permittivity, permeability and index of refraction.